The present invention relates to a bearing device used in an electric motor employed for driving a computer hard disk having a high recording density and capacity.
A bearing device in a spindle motor for driving a hard disk of a type earlier designed by the present applicant (not prior art) is arranged as shown in FIG. 1. Data signal recording media having a high recording density and capacity, namely, magnetic disks 17, are mounted on a hub 6 fixedly fitted on one end portion of the rotary shaft of the spindle motor. A bearing 33 is fixedly positioned in a bearing holder 9 with a retainer 32. A coil spring 36 is positioned with a retainer 35 in such a manner that it is held between the retainer 35 and another bearing 34 which is movably fitted in the bearing holder. The coil spring 36 applies pressure to the bearing 34 to minimize the amount of play between the balls and the inner and outer races. A rotor case 29 is fixedly mounted on the other end portion of the rotary shaft so that the inclination of the shaft and accordingly the inclination of the magnetic disks is minimized. A magnetic seal mechanism including a magnet 16 held between two yokes 14 and 15 and a magnetic fluid 17 is provided at the upper end portion of the bearing holder so that metal powder formed by abrasion of the bearings is prevented from being scattered to the outside. A shield yoke 37 is fixedly fitted on the upper end portion of the bearing holder to prevent the magnetic disks from being affected by the magnet 16.
In this bearing device, the bearing holder is made of aluminum or steel (SUS 303), the bearings are made of bearing steel, and the shaft is made of steel (SUS 416), for instance. That is, these components are formed from different materials, and accordingly they have different thermal expansion coefficients. As a result, as the temperature of the bearing device changes, gaps are formed between the components, the shaft is inclined, and the magnetic disks are tilted. Furthermore, the shield yoke 37 and the retainers 32 and 35 have smaller thermal expension coefficients than the bearing holder. Therefore, as the temperature of the bearing device changes, the bearing holder, being locally strained, tends to deform. As a result, it is difficult to maintain the recording reproducing heads in stable sliding contact with the magnetic disks; that is, it is impossible to perform high density recording and errors are liable to occur during recording.